Daily Archives: February 27, 2014

Local Meteorology and Air Quality

During certain periods, the air here at Bodega Marine Lab (BML) has been stunningly clean — approaching what might be found in the remote ocean.  We’re finding, somewhat unsurprisingly, that local meteorology heavily affects the composition of particles that we sample at BML.

Here’s a case study from last weekend.  The scene: nothing but Pacific Ocean to our west, with cow pastures, campgrounds, and a popular vacation town to our east and south.  On sunny days under fair weather (high pressure) we experience a diurnal cycle of sea breeze and land breeze.  In aerosol particle terms, we see a very segmented signal: distinct ‘sea breeze’ and ‘land breeze’ aerosol characteristics.  Let’s have a look at the aerosol particle size distributions next to the surface meteorological conditions (from the nearby DRI met station on Bodega Marine Reserve):

SeaBreezeCycle_BBY

Two days of aerosol particle size distributions (top two panels), carbon monoxide and solar radiation (3rd panel from top), ambient temperature and RH, with wind speed and direction on the bottom panel. The correlation of wind direction with the number concentration of small particles (2nd panel from top) is striking.

The sources of particles in air masses that we sample during the sea breeze (wind from 300 degrees) are ideally very limited: particles ejected from the ocean as sea spray, and particles that form through chemical reactions in the atmosphere just above the ocean.  In contrast, the land breeze (wind from about 140 degrees)  carries with it particles from a wide variety of sources: fossil fuel combustion, biomass combustion (fires), food cooking, and agriculture.

If you aren’t familiar with this kind of data, and you have a keen eye, you’ll start to figure out what’s happening just by thinking about what particle sources we’re sampling under the land breeze vs the sea breeze.  Since the sea breeze contains sea spray, you can see that sea spray aerosol particles are typically characterized by larger particle sizes (the colors in the top plot show higher concentrations near the 1 micron diameter mark), compared to the kinds of aerosols observed during the land breeze episodes (smaller proportions of larger particles compared with to the huge number of small particles).  It’s well known to atmospheric chemists that particles that are emitted directly from combustion sources (so-called ‘primary’ combustion aerosol) are very small, and tend to grow as they react with other trace gases in the atmosphere or are incorporated into clouds and fogs.

I can tell you anecdotally, that the composition of the particles is very different between these two episodes.  Sea salt particles (containing mainly sodium chloride) that travel through polluted air quickly react with the nitrogen oxide pollutants to form sodium nitrate particles.  The land breezes also bring with them fresh combustion particles.  You’ll notice some small spikes in carbon monoxide on either end of the sea breeze periods, just before and after the switch — those are actually from fires from local campgrounds directly upwind of us — it was a weekend in a vacation destination town after all!

One really exciting thing for a atmospheric chemist is to see their science in the world around them.  To live in and around the environment; to observe with your own senses what you’re observing with your instruments.  This is why I’m an environmental scientist.

-Doug Collins, Grad Student (UCSD)

Advertisements

Round 2 is on its way

Greetings from Bodega Head!

After a good soaking yesterday, the sun has come out for a brief hello.

 

Recap of Wednesday’s rain (round 1)

We received around 2 and a quarter inches of rain here yesterday, largely between 7 am Wednesday morning and 8 pm Wednesday night. A frontal boundary passed us around 7 pm Wednesday. It wasn’t a classical cold front, which can be seen from the 449 MHz wind profiler and RASS timeseries:

HMT-BBY_UTvFlux_1900UTC0227

Time-vertical cross-section of wind and virtual temperature from 449 MHz radar + RASS at BBY for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

As can be seen in the wind profiles, winds turned from SSE to SW after the boundary, and the lower atmosphere actually warmed. As we will see, freezing levels did not change much after the boundary either.

The profiling radars at Cazadero (CZC) and Santa Rosa (STR) capture this beautifully:

HMT-CZCSNR_19UTC0227

Time-Vertical cross section of signal-to-noise ratio from the S-Band Radar at CZC for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

HMT-STRSNR_19UTC0227

Time-vertical cross section of signal-to-noice ratio from the S-Band Radar at STR for the period ending at 11 am PST on Feb 27, 2014. Credit: NOAA-HMT.

For each case, the bright band does not ascend or descend after the time of frontal passage. It is apparent that CZC received a period of very heavy rain between 6 pm and 8 pm, near the frontal boundary. In fact, cloud echo-top heights reached to nearly 9 km AMSL. The wind profiler here at BBY shows that this period coincided with a dramatic increase in upsolpe water vapor flux:

HMT-BBY_UiwvFlux_1900UTC0227

Time-vertical cross section of wind and timeseries of upslope IWV at BBY for the period ending at 11 am PST on Feb 27, 2014. Credit NOAA-HMT.

In this case, the upslope wind direction is defined along the vector which points from BBY to CZC. as Southerly winds really sped up near 6 pm yesterday, upslope IWV also shot up, and for the period where the value exceeded 20 knot-inches, precipitation was enhanced at CZC compared to BBY.

Potential for dust in precip. samples:

Yesterday in this post, I mentioned that we observed dust at the surface for most of the morning, while the storm was coming ashore. We also expected high altitude dust in the storm’s central circulation. There are a few ways that elevated dust could reach our site at the ground. 1) Dust aerosol could be scavenged by falling raindrops, which then make it into our precipitation sampler. 2) Dust aerosol could act as cloud condensation nuclei, which grow into or become incorporated in a rain-drop or precipitating ice and make it into our precipitation sampler. 3) Dust aerosol could act as ice nuclei, which grow into ice-phase precipitation particles, melt on their path to the surface and make it into our precipitation sampler.

We are most interested in the last possibility, which means that we would really like to collect dust during periods when the precipitation profiling radar is telling us that there is cloud ice over our heads (i.e. a brightband is present) in a deep layer.

Even at STR, whose radar likely mimicked the cloud structure that existed over us at BBY, there were long periods of high cloud-top echoes over brightband rain yesterday. Given that we expected a deep tropospheric layer of Asian dust, particularly in the early phases of this storm, that is good news for our precipitation residue analysis!

Looking ahead to Round 2:

Since late yesterday evening we have entered an interlude between storms. Winds were southwesterly for a short time, but will soon become southerly in advance of the next system. We are going to experience a rare event here on the Northern California coast. We are going to see a small but powerful extratropical cyclone whose circulation center passes to our south. We expect the low pressure center to come onshore early Saturday morning near Monterrey Bay. Before this time, we will encounter another frontal boundary here at BBY. According to model forecasts, a mesoscale band of strong SE winds will cross the coast near 4 am PST tomorrow morning:

MM5-10mU_12UTC0227-12UTC0228

10m wind speed and wind barbs forecast by 4 km MM5 valid at 4 am PST on Feb 28, 2014. Credit: DRI-CEFA.

After this time, winds will shift to SE, then rotate to S, then E, then eventually NE and Northerly as the low pressure center comes fully onshore. We expect high wind speeds at several points tomorrow, as the particular series of events mimics a rainband, then near-miss of a tropical storm. Pretty unique for the west coast!

The bottom line is that it should start raining again tonight at around 4 am and we could have periods of rain through 1 pm Saturday. We may pick up 1-2 additional inches of rain here.

-Andy